(1) Field of the Invention
The present invention relates to a projection method and a projector with a single polarizer and a color switch.
(2) Description of the Prior Art
Nowadays, each projector is required to be capable of providing a good contrast and a good contrast while satisfying the needs of further compactness and lightweight.
Normally, in order for the luminance of the picture to be raised, it is necessary to raise the luminance of the light source lamp. Raising the luminance of the light source, however, has a limitation, since a higher luminance inevitably leads to a larger light source lamp, which results in the increase in the dimensions of related internal parts and their housing. Further, simply raising the brightness of the light source lamp gives rise to a problem that the xe2x80x9cblack-color abnormalityxe2x80x9d occurs owing to the increase in the luminance of the black color caused by the leak of the light.
A conventional single polarizer type projector comprising an optical system 10 and a circuit system 11 is given in FIG. 8.
The optical system 10 will be described first.
As shown in FIG. 8, a light source unit 12 includes a light source lamp, an integrator lens, polarized light converter, a beam-condensing element or the like. About 75-85% of the light generated by the light source unit 12 is composed of S-wave (a light component oscillating vertically to the incidence plane), and most of P-wave (a light component oscillating parallelly to the incident plane) is eliminated while passing through the polarizer 13. Of the light substantially composed of the S-wave, the direction of polarization of the bandwidth of one of R, G and B lights can be rotated by a color switch 14.
The color switch 14 comprises 3 pieces of transmission liquid crystal panels respectively for R, G and B lights and phase difference plates, which are respectively designed to rotate only a polarized light having a predetermined wavelength bandwidth, as illustrated in FIG. 9, and are superposed on one another. When the R light has to be created by using the color switch 14, only the polarized light having the wavelength bandwidth of the R light is rotated to obtain a P-wave light, while keeping the wavelength bandwidths of the G and B lights unrotated to maintain the S-waves thereof, whereby the polarized G and B lights are reflected by a polarized beam splitter prism (hereinafter referred to as BPS) 15, while only the polarized R light is transmitted to reach a reflection display panel 16. Of the light of P-wave that has arrived at the reflection display panel 16, only the picture element for display is rotated to be converted into the S-light wave and reflected. Of the light reflected by the reflection display panel 16, only the S wave is reflected in the direction bent by 90xc2x0 to be finally projected on a screen.
Further, in the case of the reflection display, the sticking due to the memory effect occurs because of the characteristic of the liquid crystal panel, unless the polarity of voltage is changed. Therefore, in order to prevent the black color and white color in a displayed image from being reversed when the polarity is set positive, a doubler 17, comprising a polarizing switching element, is provided before the reflection display panel 16.
Next, the circuit system shown in FIG. 8 will be explained.
The video signal inputted from outside is converted into a picture element suitable for the reflection liquid crystal panel 16 by means of an image processing circuit 18 and corrected by xcex3 correction circuit for the color suited for the display on the reflection display panel 16. The video signal that has undergone such a processing is displayed on the reflection display panel 16 by means of a panel drive circuit 20. In the case of a single-panel projector, as shown in FIG. 1, the display of color image can be made possible by making the color switch 14 by the timely switching of R, G and B and simultaneously by sequentially displaying the R image, G image and B image on the reflection display panel 16.
Therefore, trigger signal from a panel drive circuit 20 is sent to a color switch drive circuit 21 so that the color switch 14 is made to switch the color to be displayed synchronizing with the display on the reflection display panel 16.
The timing of the transmission of the trigger signal from the panel drive circuit 20 to the color switch circuit 21 and that of the transmission of color switch drive signal from the color switch drive circuit 21 to the color switch 14 are shown in FIG. 11. The negative side of the color switch drive signal amplitude serves for preventing the sticking due to the memory effect from occurring on the liquid crystal reflection display panel because of the characteristic thereof, unless the polarity of the impressed voltage is kept unchanged.
In FIG. 11, (a) represents a vertical synchronizing signal; (b) a condition in which R, G and B signals are switched in a reflection display panel; (c) a switch trigger signal outputted at the fall of the vertical synchronizing signal; (d) a condition in which the cell is in off-state when the voltage of each cell (liquid crystal) of the color switch is UV, while the cell is in on-state when the voltage of each cell is VL.
In the case of a conventional single polarizer projector, the color switch 14 is used only for the switching of 3 patterns, i.e., R, G and B, and the reflection display panel 16 is used for the tonal expression of the image. More particularly, since the transmittance of the color switch 14 is always kept constant, if the quantity or intensity of the light is increased in order to raise the brightness of an image, even the degree of the darkness becomes weaker due to the effect of the leaking light, resulting in the separation of the dark portion of the image and insufficient contrast.
An object of the present invention is to provide a projection method and a projector not only capable of suppressing the poor black color matching resulting from raising the luminance of the power source lamp but also capable of raising the contrast.
Further, the present invention relates to a projection method and a projector, the projector comprising a single polarizer projector and a color switch, wherein the luminance of the video signal is calculated; the transmittance of the color switch is controlled according to the mean luminance; the transmittance is set to 100% (a maximum) for bright image; the transmittance is lowered for dark image to prevent the poor black color matching in the case of dark image, thereby improving the contrast of the image.
With respect to a mean luminance calculation circuit for calculating the luminance of the video signal, outputted from an image processing circuit, in converting the R, G and B signals into Y signal respectively, the luminance signal is generated according to an equation, Y=0.30R+0.59G+0.11B, which is defined by the NTSC standard.
In converting the R, G and B signals, outputted from an image processing circuit, into Y signal respectively, by the mean luminance calculation circuit for calculating the luminance of the video signal, the scale of the circuit can be reduced by generating the luminance signal by using the equation, Y=0.3125R+0.5625G+0.12500B, a modification of the equation according to the NTSC standard, using the coefficient which can be realized through the bit shift calculation and addition.
With respect to the mean luminance calculation circuit for calculating the luminance of the video signal, the calculation of the mean luminance signal APL is simplified by counting the carry of an adder and by subsequently counting the carry of the counter when converting the R, G and B signals, outputted from the image processing circuit, into Y signal respectively and adding the Y signals during 1V period.
With respect to a color switch drive circuit for driving color switch, both the poor luminance and the blackening of low-luminance image can be suppressed by setting transmittance to 100% when the APL signal is 50% or more while setting the lower limit of the same to 50% when the APL signal is below 50% in determining the transmittance of the color switch according to the APL signal outputted from the mean luminance calculation circuit.
With respect to the color switch drive circuit for driving the color switch, the color switch, whose transmittance varies according to the drive voltage, is controlled by selecting a proper transmittance data of the color switch by using a memory such as a look up table (LUT) when determining the transmittance of the color switch according to the APL signal outputted from the mean luminance calculation circuit, and by converting the transmission data into analog data through a D/A.
With respect to a transmittance setter of the color switch, both the poor luminance and the blackening of a dark image can be suppressed by varying the transmittance, e.g., by setting the transmittance to 100% when the APL signal is 50% or more while setting the lower limit of the transmittance to 50% when the APL signal is below 50% by means of a circuit comprising a combination of several OR gates, in setting the transmittance of the color switch according to the transmittance of the color switch according to the APL signal outputted from the mean luminance calculation circuit.
The occurrence of the blackening can be prevented by controlling the transmittance of the color switch, and by correcting dynamic xcex3 so that the dynamic xcex3 has a larger inclination towards a lower level when the luminance of any given image is relatively low.
The white balance and the color tone are adjusted by separately controlling the transmittances of the R, G and B of the color switch according to the white balance adjusting signal and the color tone adjusting signal outputted from a microcomputer or the like.
As for the color switch, the light, as being a mixture of the P wave light and S wave light, is polarized by a PBS and a polarizer provided before the PBS so that the S wave light to fall on the reflection display panel after passing through the PBS is substantially removed, thereby improving the contrast.
The light to be projected against the screen from the PBS is polarized by an absorption polarizer having a higher polarizing efficiency to remove the P wave and to thereby improving the contrast.